Since I have to guess here, I would say that your old project used .mov Quicktime movie files that require QuickTime as a system driver for time based media.

Maybe.

Hopefully, your .mov files are now supported by Premiere Pro directly and just need a little nudge to work as expected. For example, I've had old projects with DV-NTSC source footage that didn't have ".mov" in the filename extension. Adding that to each filename allowed Premiere Pro to import and play the file as expected. I've also run into .aiff audio files needing to be renamed to ".wav" for some sound files to work as they used to.

Again, this is just a guess. Even if it's not QuickTime specific, you're probably dealing with something that changed that allows your hardware to support the picture of your source footage.